add how-to guide for flashing firmware with esptools

[capsulecorplab]

a how-to guide containing:

• [x] - steps for cloning or downloading & extracting from release page
• [x] - (link to) instructions for installing esptools.py
• [x] - steps for flashing firmware onto nugget

[AlexLynd]

Done! We just pushed that documentation yesterday here - https://usbnugget.com/docs/guides/update-with-esptool/

We're working on making the documentation more coherent, but if you have any suggestions / requests, you can leave them on our repo.